The invention relates to a process for the cooling of cryocables by means of low-boiling gases.
In the transmission of high electric power, the resistance losses in the transmission cable become so great that overheating of the cable is common. For many years attempts have been made to construct cables which are suitable for the transmission of high electric power. The construction of such high-power cables in all instances attempts to provide the cables with efficient cooling. Liquefied, low-boiling gases, among other things, are intended as the coolants for this purpose.
Cables cooled with such cryogenic liquids are hereafter called cryocables. Two types of cryocables are distinguished including the superconducting cables which relate to maximum-power cables and high-power cables which utilize the resistance reduction of normally-conducting metals with lowering temperatures.
In the case of superconducting cables wherein the superconducting cable core must be cooled below the particular transition temperature, the cryogenic liquid, e.g. helium, is absolutely necessary. With high-power cables having copper or aluminum as the conductor, advantages are brought forth by the use of lower temperatures on the basis of the strong temperature dependence of the specific electric resistance. Thus, for example, the specific electric resistance of pure copper in cooling from room temperature to 20.degree. K is reduced by almost a factor of 10.sup.-.sup.4. Here the temperature dependence of the specific electric resistance between 20.degree. and 30.degree. K is particularly great.
The advantages offered by the cryocables with respect to the transmissible electric power are counteracted by the drawback of a relatively great expenditure necessitated by the special requirements of the low-temperature techniques. Thus, it is necessary to provide cooling stations at certain intervals along the entire length of the cable wherein the cryogenic coolant is recooled after being warmed. This warming is unavoidable with superconducting cables since even with the best insulation heat is absorbed from the surroundings. Furthermore, pumping stations must be distributed at certain intervals along the entire cable length in order to equalize the pressure loss experienced by the cryogenic coolant on passing through the cooling channel along the cryocable.
The coolant for the cooling of cryocables may be a low-boiling, liquefied gas in a boiling equilibrium. In this case the cooling liquid enters the conduit at an equilibrium temperature relating to the pressure. Any heat admission, be it from inside or outside, then leads to the formation of saturated steam so that with a growing distance from the inlet, the steam proportion continues to grow. Also, a recooling station must be provided before or at the point where the entire cooling liquid is evaporated to supply fresh liquid to the cable.
It is more advantageous if the pressure in the cryocable is raised to such an extent that the steam formation is fully eliminated. The liquid in this case is undercooled and the heat absorption leads to a rise in the liquid temperature. A recooling station must be then provided before or at the point where the temperature of the liquid has risen due to the heat supply to such an extent that the particular saturated steam pressure corresponds to the pressure in the cooling system. Such a system has the advantage of a reduced safety risk, since in a breakdown of the pumping or recooling station, the liquid may still evaporate and the cooling of the cable may still be continued for a period of time before damaged due to excessive heat.
A considerable part of the expenditures required for a cryocable involves the recooling stations. It is obvious that a cryocable becomes increasingly more expensive as the distance between recooling stations decreases.